Tricyclic compound 1 [TBE-31, (±)-(4bS,8aR,10aS)-10a-ethynyl-4b,8,8-trimethyl-3,7-dioxo-3,4b,7,8,8a,9,10, 10a-octahydrophenanthrene-2,6-dicarbonitrile, FIG. 1] is one of the most potent activators of the Keap1/Nrf2/ARE pathway known to date (Honda, T. et al. 2007; Honda, T. et al. 2011; Liby, K. et al. 2008; Dinkova-Kostova, A. T, et al. 2010). Tricyclic compound 1 suppresses pro-inflammatory responses and induces heme oxygenase-1 (HO-1) in RAW264.7 cells, and upregulates NAD(P)H: quinone oxidoreductase 1 (NQO1) in Hepa1c1c murine hepatoma cells through the Keap1/Nrf2/ARE pathway (Honda, T. et al. 2011). Incorporation of small quantities (9.2 mg per kg of food) of 1 in the diet of mice profoundly and dose-dependently induces NQO1 and glutathione S-transferases in the stomach, skin, and liver (Dinkova-Kostova, A. T. et al. 2010). Long-term (five days per week for four weeks) topical daily applications of 200 nmol of 1 causes a robust systemic induction of the Keap1/Nrf2/ARE pathway and decreases the 6-thioguanine incorporation into DNA of skin, blood, and liver of azathioprine-treated mice, indicating extraordinary bioavailability and efficacy (Kalra, S. et al. 2012). Tricyclic compound 1 has two different monocyclic nonenolizable cyanoenones 2 and 3 (FIG. 1) in rings A and C. It has been chemically demonstrated that rings A and C of TBE-31 give reversible Michael adducts with the sulfhydryl groups of Keap1 and dithiothreitol (DTT) by UV spectroscopy studies (Liby, K. et al. 2008; Dinkova-Kostova, A. T. et al. 2010).
Generally, there are three categories of drugs: (1) Irreversible covalent drugs, which permanently bind with protein targets through covalent bonds. This category includes alkylating agents (e.g., cyclophosphamide, mitomycin C), β-lactam antibiotics (e.g., penicillin, cephalosporin) and irreversible enzyme inhibitors (e.g., acetylsalicylic acid, omeprazole); (2) Reversible non-covalent drugs, which non-covalently (e.g., hydrogen bond, hydrophobic effect and Van der Waals forces) bind with protein targets. This category contains receptor antagonists (for example, angiotensin II receptor blocker, β-blockers, and histamine H2-receptor antagonists etc.); and (3) Reversible covalent drugs, which covalently bind but not permanently with protein targets. This final category is a new category. Currently, dimethyl fumarate (DMF) (Gold, R. et al. 2012), which is a Michael acceptor and an activator of the Keap1/Nrf2/ARE pathway, is the only drug that is clinically used for the treatment of multiple sclerosis. CDDO and bardoxolone methyl also belong to this category. Recently, the development of a series of reversible covalent inhibitors of MSK/RSK-family kinases has been reported (Miller, R. A. et al. 2013).
Although irreversible covalent drugs have long duration of action, high potency and high ligand efficiency, because they irreversibly bind to both on- and off-protein targets, there is a potential for immune-mediated hypersensitivity and therefore they are not suitable for chronic dosing. Reversible non-covalent drugs do not form permanent adducts and therefore are suitable for chronic dosing. However, their selectivity and potency are moderate because their ligand efficiency is usually poor. To the contrary, reversible covalent drugs have high potency, high ligand efficiency and long duration of action and because they do not form permanent adducts, they are suitable for chronic dosing. Overall, reversible covalent drugs combine the advantages and circumvent the disadvantage of irreversible covalent and reversible non-covalent drugs. Nevertheless, reversible covalent drugs have been largely ignored because of the lack of reactive compounds to produce the reversible covalent adducts with protein targets.